Combined superplastic forming /diffusion bonding (SPF/DB) is an established technique for making structural components, particularly lightweight components requiring complex internal structures, from materials that exhibit superplastic properties at elevated temperatures. These materials are primarily titanium alloys, especially (but not exclusively) titanium/aluminium/vanadium alloys.
Typical examples of known superplastic forming/diffusion bonding processes are described in U.S. Pat. No. 5,143,276, U.S. Pat. No. 4,534,503, GB-2,030,480, GB-2,129,340, U.S. Pat. No. 4,607,783U.S. Pat. No. 4,351,470, U.S. Pat. No. 4,304,821, U.S. Pat. No. 5,994,666 and EP-0,502,620.
A typical known SPF/DB process will now be described in connection with FIG. 1, which shows four steps for forming a panel. In step 1, stopping off material may be applied between two core sheets 10, 12; stopping off material forms a layer that prevents the core sheets being diffusion bonded together at operating temperatures in the areas to which the stopping off material has been applied. The core sheets are then joined together by line bonds 14. These bonds can be formed by diffusion bonding the two core sheets 10,12 together, in which case the stopping off material should be omitted in the region of the line bonds 14. Alternatively, the core sheets can be bonded together by other techniques, for example resistance welding or laser bonding.
In step 2, a pack assembly is formed by sandwiching the core sheets 10, 12 between skin sheets 16, 18; the pack may then be sealed around its outer perimeter by a weld or a bond (not shown). Ducts are included in the pack assembly allowing gas to be injected into the region between the core sheets 10, 12 and independently in the region between the skin sheets 16, 18 and their adjacent core sheets, 10, 12. If necessary, gaps can be left in the line bonds 14 to allow the passage of gas between adjacent regions of the core sheets.
In step 3, the pack assembly is then placed between two halves of a moulding tool 20 that can be heated. The two halves of the moulding tool 20 are pressed together to form a gas tight seal between the edges of the pack assembly and the internal cavity in the tool. The clamping forces, when subsequently supplemented by heating, can provide for the development of diffusion bonds 21 around the periphery of the pack if so desired.
The tool is heated to a temperature at which superplastic forming takes place, which is typically in excess of 850° C. for a typical alloy, such a Ti-6% Al-4% V. An inert gas is firstly injected between each skin sheet 16, 18 and its adjacent core sheet 10, 12 respectively. This causes the skin sheets 16, 18 to be urged against the internal face of the mould tool 20, thereby adopting the shape of the internal face of the mould tool 20. Once the skin sheets 16, 18 have been urged away from the core sheets 10, 12, e.g. once they have been partially or fully formed into shape by the tool 20, gases are injected between the core sheets 10, 12 causing the areas between the bonds to “inflate”. The inflation continues until the core sheets form a series of cells 22 divided by walls 24. The upper half of each wall 24 is formed by a double-backed section of core sheet 10; likewise, the bottom half of each wall 24 is formed by a double-backed section of core sheet 12. The bonds between the two halves of the wall are the line bonds 14 formed in step 1.
In step 4, the gas pressure within the cells 22 is maintained for a time after the cells have been inflated to form diffusion bonds 28 between the skin sheets 16, 18 and the adjacent areas of the core sheets 10, 12. Likewise, diffusion bonds 28 are formed between the double-backed sections of the core sheets 10,12 forming the walls 24 and between the outer edges 26 of the outer perimeter of the pack compressed by the two halves of the moulding tool 20.
The strength of the panel is greatly enhanced by the presence of the diffusion bonds 28 and it is desirable that they should be formed at all interfaces between the core sheets and the skin sheets. To that end, the gas within the cavities 30 between the core sheets and the skin sheets is controlled and gas is withdrawn from the cavities as they shrink during inflation of the cells to prevent the gas being trapped between the core and skin sheets, which would prevent intimate contact between these sheets and so hinder diffusion bonding. Gas is withdrawn from the cavities 30 in the region of the spandrels 32 formed at the top and bottom of the walls 24 between the core sheets and the skin sheets.
At superplastic forming temperatures, titanium alloys can form a surface layer (or “case”), which is an alpha phase formed particularly in the presence of alpha phase stabilising elements, such as oxygen and nitrogen. The formation of an alpha case in a location that is to be diffusion bonded drastically reduces the strength of the diffusion bond and in addition has a detrimental effect on fatigue performance. For this reason, the gas used in superplastic forming should be substantially free of such alpha case stabilising elements and so a high purity gas with a very low content of alpha case stabilising elements (in excess of 99.999% purity) should be used. In addition, the gas is customarily passed over a “getter” to further reduce the amount of any impurities that may be present. The gas that is almost universally used in superplastic forming is argon because it is inert and relatively cheap. Other inert gases have not been used since there has been no perceived advantage in using them over and above argon.
During the inflation of the core sheets, when the core sheets first contact the skin sheets (so-called “sticking contact”), there is a tendency for a compressive stress to be imparted by the expanding core sheets to the skin ahead of the advancing sticking contact point. This progressive stress can cause buckling of the skin layer, which is unsupported ahead of the advancing sticking contact point. The development of such buckling eventually causes excess skin material to be drawn into the cell structure at the point of the cell boundaries (i.e. above the spandrels) and a line defect in the skin occurs above the spandrels. It is customary, in order to minimise skin buckling, to maintain a back pressure of gas in the cavity 30 between the core sheet and the skin sheet during inflation of the cores. The magnitude of the back pressure necessary to avoid such buckling depends on the relative thickness of the core and the skin sheets and the geometry of the cells. The back pressure is normally removed once the cores have been fully formed (or approaching being fully formed) in order to prevent gas being trapped between the core sheet and the skin sheet, which reduces the strength of the diffusion bond between these sheets or indeed can prevent a diffusion bond being formed in those areas where gas is entrapped. Gas is usually removed from the cavity between the core and skin sheets via the spandrels, which maintains a gas conduit for at least a time after the core cells have been substantially formed. Thus, the removal of the back pressure between the core and skin sheets minimises the degree of potential gas entrapment within the spandrel structure that may result if the spandrel network should subsequently become blocked.
A schematic pressure-time cycle (PTC) in respect of the inflation of the core sheets is shown in FIG. 2. FIG. 2 does not include a PTC in respect of the inflation of the skin sheets. As can be seen, a back pressure (dashed line (-----) “a”) is maintained between the core sheets and the skin sheets during inflation of the core sheets (step 3, indicated by arrow “3”) but, once the core cells 22 have been substantially formed, the back pressure is removed and the pressure within the core cells is maintained for a predetermined time to allow for diffusion bonding within the panel. The pressure in the cells 22 is indicated by chained line () “b”, giving a net pressure across the core sheets 10,12 indicated by solid line() “c”.
The quality of the diffusion bonds formed during and after superplastic forming can be adversely affected by the use of a back pressure of gas in the cavity between the core sheets 10,12 and the skin sheets 16,18 caused, it is believed, by entrapment of small pockets of gas during the SPF/DB process. This is true even in the case of a PTC shown in FIG. 2 where the backing gas is evacuated as the cores are formed and the cavity between the core sheet and the skin sheet reduces in size. FIG. 3 shows a photomicrograph through the diffusion bonded region between a core sheet and a skin sheet using the above-described SPF/DB process. The black areas show entrapped gas. The rounded nature of the ends of the malformed bonds (“disbonds”) is characteristic of there having been gas entrapment preventing intimate contact from occurring. The absence of any alpha case at the bond line confirms that the surfaces of both layers were clean during diffusion bonding.
Without wishing to be committed to any particular theory, it is believed that the gas is trapped as a result of high levels of strain-induced surface roughness. During superplastic forming, the high level of strain is accommodated by the material of the sheets by a process known as “grain boundary sliding”, that is to say individual grains within the metal slide past each other during superplastic forming. The inevitable result of grain boundary sliding is that the surfaces of the sheets become roughened at a microstructural level due to individual surface grains protruding out of the original planar surfaces of the sheets being formed. As the surfaces of the core and skin sheets are brought into intimate contact under the application of the bonding pressure within the cores, any previously roughened surfaces will deform to produce an essentially flat interface. However, it is believed that gas can become trapped in the crevices behind protruding grains and become isolated from the receding cavity that will eventually become the spandrel. Without such a vent path back to the spandrel, a pocket of gas forms and prevents diffusion bonding. The application of back pressure compounds the above problem since a greater quantity of gas will be present in the cavity between the sheets to be diffusion bonded.
WO02/22286 describes a method of superplastic forming a single sheet using a silica mould. In order to prevent excessive contact between the sheet and mould, which could contaminate the sheet, a barrier is formed between the sheet and the mould, which may be solid or gaseous, e.g. boron nitride or an inert gas such as helium or argon.
U.S. Pat. No. 4,500,033 discloses a method of expelling entrapped air during superplastic forming by coating the superplastic sheets with a material that decomposes at a temperature below superplastic forming temperature to form an inert gas. The decomposition gas is then flushed out together with entrapped air by means of argon.